The present invention relates generally to assembly tools and methods of manufacture associated with the sequential assembly of work-pieces. More particularly, this invention pertains to managing assembly tool travel in moving work-piece assembly manufacturing systems such as moving conveyor systems or automatic guided vehicle (AGV) systems. Even more particularly, this invention pertains to limiting the travel of electrical, pneumatic or hydraulically powered assembly tools used within station work envelopes of a moving work-piece manufacturing system.
FIG. 1 shows a portion of a moving work-piece assembly manufacturing system 10. The example shown is of a typical vehicle manufacturing system using a conveyor system 11 to transport work-pieces 70 within a conveyor footprint 12 along a direction of travel 13. A conveyor footprint 12, as used herein, is the physical space traversed by a conveyor system 11 and work-pieces 70 transported by the conveyor system 11.
In conventional assembly line manufacturing, work-pieces under assembly are moved into a work station, work station specific assembly operations are performed on the work-pieces and the work-pieces are moved out of the work station for any needed further assembly operations. Typically, work stations are organized along the conveyor footprint in generally single or branched path linear configurations according to the predetermined sequence of assembly operations to be performed upon the work-pieces. However, non-linear path assembly line configurations may also employ similar work stations.
A work station, as used herein, is the designated physical space within which a designated assembly operation is performed upon a work-piece positioned within the space. A work station envelope is the perimeter of the physical space of the work station. A work station envelope may be defined in one, two or three dimensions as necessary to practically delimit the work station envelope within an assembly line configuration. For example, FIG. 1 shows the overhead plan of a linearly configured portion of a vehicle manufacturing assembly line. Work station envelope 22 is defined along the direction of travel 13 by the intersection of work station forward boundary 26 with the conveyor footprint 12 and by the intersection of the work station rear boundary 28 with the conveyor footprint 12. FIG. 1 shows in-station work-piece 71 within work station envelope 22 and out-of-station work-pieces 72 outside of work station envelope 22. Since the work-pieces 70 move in a linear fashion along conveyor footprint 12 of this assembly line, there is no need to precisely define the perimeters of work station envelope 22 in the directions orthogonal to the direction of travel 13.
Work stations have at least one associated assembly tool 40. Each associated assembly tool 40 is designed to perform a designated assembly operation upon a work-piece 71 positioned within the work station envelope 22. Each designated assembly operation is performed at a designated point-of-use 75 located on each work-piece 70. In many critical assembly operations the point-of-use 75 is a unique point on each work-piece 70. For example, a designated assembly operation may be torquing the pinch bolt for a steering column yoke. In order to ensure that the proper bolt is torqued, the bolt may have a uniquely configured torquing means, such as a uniquely shaped or sized bolt cap. The associated assembly tool 40 would have a correspondingly configured torquing means, such as a torque socket uniquely shaped and sized to securely receive the unique bolt cap. This ensures that the associated assembly tool 40 in work stations 20 designed to perform a critical assembly operation can only be used at the designated point-of-use 75 on each work-piece 70. Herein, it is assumed that associated assembly tools 40 in work stations 20 can only be used at the designated point-of-use 75 on each work-piece 70.
Assembly tools 40 may be of various types and configurations depending upon the required assembly operations and the method of assembly. Assembly tools may be manual, semi-automated or fully automated. In many conventional assembly line operations, such as shown in FIG. 1, the assembly tools 41 of work stations are semi-automated. These semi-automated tools 41 are characterized as being powered tools requiring at least some operator manipulation to position the tool to its point-of-use 75 upon the work-piece 70, 71 and operator manipulation to activate the tool. Powered assembly tools 41 may consist of a power and control unit 42 connected to an end-use device 43 by a flexible power transfer section 45. One typical end-use device 43 is a torque driver. Typical flexible power transfer sections 45 are power transfer conduits 46 such as electrical control cables, cords, and pneumatic or hydraulic hoses.
Work stations assembly tools are frequently limited in their travel. Assembly tools may be tethered by flexible structures such as a cord or cable, or by articulated structures such as manipulator arms. Tethers may be elastic or non-elastic and may include: power transmitting tethers such as electrical control cables, electrical power cables, or pneumatic or hydraulic hoses connecting a drive unit to an end-use device; non-power transmitting tethers such as cords, chains, wires, or cables; or combinations thereof. Assembly tool flexible power transfer sections 45 such as electrical control cables, electrical power cables, and pneumatic or hydraulic hoses are the most common form of tether.
These electrical control cables, electrical power cables, and pneumatic or hydraulic hoses are usually supplied by the assembly tool 40 manufacturer and are of standard lengths. The point of attachment of these flexible power transfer sections 45 is a pivot point 53. At maximum extension the flexible power transfer section 45 of assembly tool 40 is shown in FIG. 1, limiting travel of the end-use device 43 to an assembly tool travel envelope 50. The tool travel envelope 50 shown in FIG. 1 is the area within a semi-circle overlaying the conveyor footprint 12.
A tool travel envelope 50, as used herein, is the designated physical space within which the tool travel of the end-use device 43 of a designated assembly tool 40 is physically limited. A tool travel envelope 50 may be defined in one, two or three dimensions as necessary to practically delimit the tool travel envelope 50 within an assembly line configuration.
As shown in FIG. 1, the assembly tool travel envelope 50 of designated assembly tool 40 greatly exceeds work station footprint 24. Assembly tool travel envelope 50 overlaps three work-pieces 70, including the in-station work-piece 71 and portions of a upstream (relative to direction of travel 13) out-of-station work-piece 72 and portions of a downstream out-of-station work-piece 72. Each of the three work-pieces 70 has a designated point-of-use 75 associated with the work station 20. Only the in-station work-piece 71 has in-station point-of-use 75, 78 that would be the correct point-of-use for the associated assembly tool 40 to be used to perform the designated assembly operation during the time work-piece 71 is positioned within the work station envelope 22. However, it is possible for an operator to position the associated assembly tool 40 on the out-of-station point-of-use 75, 79 of the upstream out-of-station work-piece 72. Here the designated point-of-use 75 on each work-piece 70 is a single operation point-of-use 76 requiring only a single activation of the assembly tool 40. A single bolt requiring torquing would be an example of a single operation point-of-use 76.
A similar configuration and effect is shown in FIG. 2. FIG. 2 shows a second work station 21 having a second work station envelope 23, second work station footprint 25, defined along the direction of travel 13 by the intersection of second work station forward boundary 27 with the conveyor footprint 12 and by the intersection of the second work station rear boundary 29 with the conveyor footprint 12. A second assembly tool 60 is shown having a power and control unit 61 connected to an end-use device 63 by a flexible power transfer section 65. The point of attachment 56 of these flexible power transfer sections 65 is a pivot point 57. At maximum extension the flexible power transfer section 65 of assembly tool 60 limits travel of the end-use device 63 to an assembly tool travel envelope 51.
Note that the second work station 21 overlaps work station 20. This is permissible where the end use device of assembly tool 40 is configured so as not to be operable upon the designated point-of-use of the second assembly tool 60 and where the end use device of the second assembly tool 60 is configured so as not to be operable upon the designated point-of-use of assembly tool 40.
Assembly tool travel envelope 51 of designated assembly tool 60 greatly exceeds work station footprint 25. Assembly tool travel envelope 60 overlaps three work-pieces 70, including the in-station work-piece 71 and portions of a upstream (relative to direction of travel 13) out-of-station work-piece 72 and portions of a downstream out-of-station work-piece 72. Each of the three work-pieces 70 has a designated second point-of-use 74 associated with the work station 21. Only the in-station work-piece 71 has in-station points-of-use 74, 78 that would be the correct point-of-use for the associated assembly tool 60 to be used to perform the designated assembly operation during the time in-station work-piece 71 is positioned within the work station envelope 23. However, it is possible for an operator to position the associated assembly tool 60 on the out-of-station point-of-use 74, 79 of either the upstream or downstream out-of-station work-pieces 72. Here, the designated second point-of-use 74 is a multiple operation point-of-use 77 and requires multiple activations of assembly tool 60. Multiple bolts at essentially the same location that requiring torquing would be an example of a multiple operation point-of-use 77.
The configurations of FIGS. 1 and 2 and other similar configurations of the moving work-piece assembly manufacturing system 10 each allow an operator to perform the designated assembly operation on an out-of-station work-piece 72. If such an error is not noticed and corrected, the current in-station work-piece 71 may be shipped in a defective condition with a designated assembly operation not performed. If the designated assembly operation is a critical assembly operation, the safety of the work-piece may be impaired. The result of such errors may be expensive rework or costly product recalls.
Many conventional assembly line operations, such as automobile assembly lines, employ electronic monitoring systems to improve the quality control and proper performance of designated assembly operation upon the proper, in-station work-pieces 71. Conveyor systems 11 commonly use electronic sensors to monitor the movement of work-pieces 70 at various positions along the conveyor footprint 11. The work-pieces may be uniquely identified and their movement along the conveyor 11 may be recorded in a software database for quality control analysis. Conveyor systems 11 may also use interlocks that have various functions. For example, it is known to interlock the operation of an assembly tool 40 with the signal of a sensor to prevent activation of the assembly tool 40 unless it determines that a work-piece requiring the designated assembly operation is positioned within the work station 22. It is also known to limit the number of activations of the assembly tool 40 to the number necessary to perform the designated assembly operation. In many cases the operation of the assembly tool is interlocked to the conveyor and will stop the assembly line if the required number of operations are not registered as performed on the in-station work-piece within a work station. These interlocks may operate independently or may be part of an integrated conveyor monitoring and control system.
One such conveyor monitoring and control system and method is the Error Proofing System used by in certain vehicle assembly lines. The Error Proofing System is an integrated conveyor monitoring and control system in which activation of the assembly tool 40 is interlocked such that with the system only allows activation of the assembly tool 40 when activation parameters are met, herein termed system authorized activation. The Error Proofing System also limits the number of activations of the assembly tool 40 to the number necessary to perform the designated assembly operation. The Error Proofing System will stop the assembly line if the required number of operations for an authorized activation of an assembly tool 40 are not sensed as having been performed prior to the system determining that the in-station work-piece 71 is exiting a work station 20. This determination may be based on several factors including elapsed time in the work station 20 and/or sensed movement of the in-station work-piece 71.
Among other monitoring devices, the Error Proofing System uses interlocked sensors placed at reference points to monitor and control the movement of work-pieces 70 along the conveyor footprint 12. The Error Proofing System continually updates and references a software controlled database based, in part, upon the sensing of movements of uniquely identified work-pieces 70 through these reference points. One type of monitored reference point 19 used by the Error Proofing System are fixed stopping points 14.
Referring again to FIG. 1, a first fixed stopping point 15 and a second fixed stopping point 16 are shown. Fixed stopping points 14 are register shift positions for the conveyor monitoring and control system. When the Error Proofing System senses that a work-piece 70 has passed the first fixed stopping point 15, certain database registers are reset. The Error Proofing System will no longer register the activation of the assembly tool 40. Authorized activation of the assembly tool 40 may also be inhibited.
When an in-station work-piece 71 is determined to have reached the work station forward electronic boundary 17, work-piece 70 is now designated as in-station work-piece 71. Data for the work-piece 71 is now associated with data for the work station 20. The Error Proofing System will now register the activation of the assembly tool 40. Most frequently, the work station forward electronic boundary 17 is equivalent to the work station forward boundary 26, but it is not necessary that the two overlap. The operator should then position the assembly tool 40 upon in station point-of-use 71 and perform the designated assembly operation before the in-station work-piece 71 is determined to have reached the work station rear electronic boundary 18.
When the in-station work-piece 71 is determined to have passed the work station rear electronic boundary 18, the Error Proofing System will no longer register the activation of the assembly tool 40. Authorized activation of the assembly tool 40 may also be inhibited. Most frequently, the work station rear electronic boundary 18 is equivalent to the work station rear boundary 28, but it is not necessary that the two coincide. Generally, the work station rear electronic boundary 18 is also equivalent to a second fixed stopping point 14, but, again, it is not necessary that the two coincide.
When the in-station work-piece 71 is determined to have reached the second fixed stopping point 14, the Error Proofing System will allow the in-station work-piece 71 to exit the work station 20 if recorded parameters indicate that authorized activation of the assembly tool 40 occurred while the work-piece 71 was within the electronically enabled work station envelope. Otherwise, a stop interlock will stop the conveyor system 11 until the error is corrected.
As used herein, the term electronically enabled work station envelope 99 is the perimeter of the physical space of a work station within which a designated assembly operation is to be performed upon a work-piece positioned within the work station space by an authorized activation of the assembly tool and within which such authorized activation of the assembly tool will be registered by a conveyor monitoring and control system. In FIG. 1, the electronically enabled work station envelope 99 is defined along the direction of travel 13 by the intersection of work station forward electronic boundary 17 with the conveyor footprint 12 and by the intersection of the work station rear electronic boundary 18 with the conveyor footprint 12. Since the work-pieces 70 move in a linear fashion along conveyor footprint 12 of this assembly line, there is no need to precisely define the perimeters of electronically enabled work station envelope 99 in the directions orthogonal to the direction of travel 13.
FIG. 1 illustrates that the interlocks of the Error Proofing System can be defeated by simply having the assembly tool travel envelope 50 overlap an out-of-station point-of-use 79. The operator is free to position the assembly tool travel envelope 50 upon out-of-station point-of-use 79. Activation of the assembly tool 40 while thus positioned is registered as an authorized activation of the assembly tool 40 occurring while activation of the assembly tool 40 was authorized for the in-station work-piece 71. The Error Proofing System releases the in-station work-piece 71 from the work station 70 without the designated assembly operation having been performed.
An additional error occurs where the operator positions the assembly tool 40 upon in-station point-of-use 78 prior to in-station work-piece 71 reaching the work station forward electronic boundary 17. Activation of the assembly tool 40 while thus positioned is not registered. The stop interlock will stop the conveyor system 11 when in-station work-piece 71 is determined to have reached the second fixed stopping point 14. The Error Proofing System prevents the in-station work-piece 71 from exiting the work station 70 even though the designated assembly operation has been performed.
What is needed, then, is means of correcting the above identified problems.